Method for performing hierarchical beamforming in wireless access system and device therefor

ABSTRACT

A method for performing hierarchical beamforming in a wireless access system and a device therefor are disclosed. Particularly, the method comprises: an initial step for allowing a base station to transmit a plurality of first beams, to which different steering vectors are applied, to a terminal through corresponding reference signals, and a repetition step for allowing the base station to transmit a plurality of second beams, to which different steering vectors are applied, to the terminal through corresponding reference signals by considering feedback information that contains an index of one or more beams received from the terminal, wherein the repetition step can be repeated up to a predetermined number of times.

TECHNICAL FIELD

The present invention relates to a wireless access system, and moreparticularly, to a method for performing beamforming in a wirelessaccess system that supports massive multi-input multi-output (MIMO), anda device supporting the same.

BACKGROUND ART

Many schemes based on multiple antennas have been studied to improvelink quality between a transmitter and a receiver and support a highdata transmission rate in accordance with requirements of a nextgeneration wireless access system. Schemes such as space frequency blockcoding (SFBC), spatial multiplexing (SM), closed-loop MIMO(CL-MIMO)/beamforming and zero-forcing beamforming (ZFBF) have beenapplied to LTE or LTE-A system.

Generally, a mobile communication system considers that more antennasare installed in a base station than a user equipment for a reason ofphysical space and power supply, and the current LTE-A system(release-10) supports maximum 8 Tx system. Among methods for improvinglink quality by using multiple antennas, in case that channel stateinformation may be used by a transmitter, a beamforming scheme mayprovide the most excellent throughput. The beamforming scheme may obtaingain for reducing a transmission power or more improved link quality ifthe number of transmitting antennas is increased. Also, as the number ofantennas is increased, beam may be formed sharply and at the same timemore orthogonal beams may be generated. In other words, the number ofreceivers, which may receive their respective data at the same time, isincreased. In this respect, a system, which supports large scaledantennas more than the existing 8 Tx antennas, that is, massive MIMOsystem is considered.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method for desirablyperforming beamforming between a user equipment and a base station in awireless access system, preferably a wireless access system thatsupports massive multi-input multi-output (MIMO), and a devicesupporting the same.

It will be appreciated by persons skilled in the art that the objectsthat could be achieved with the present invention are not limited towhat has been particularly described hereinabove and the above and otherobjects that the present invention could achieve will be more clearlyunderstood from the following detailed description.

Technical Solution

In one aspect of the present invention, a method for performinghierarchical beamforming in a wireless access system comprises aninitial step for allowing a base station to transmit a plurality offirst beams, to which different steering vectors are applied, to a userequipment through corresponding reference signals; and a repetition stepfor allowing the base station to transmit a plurality of second beams,to which different steering vectors are applied, to the user equipmentthrough corresponding reference signals by considering feedbackinformation that includes indexes of one or more beams received from theuser equipment, wherein the repetition step may be repeated as much as apredetermined number of times.

In another aspect of the present invention, a base station forperforming hierarchical beamforming in a wireless access systemcomprises a radio frequency (RF) unit configured to transmit and receivea radio signal; and a processor, wherein the processor is configured toperform an initial step for transmitting a plurality of first beams, towhich different steering vectors are applied, to a user equipmentthrough corresponding reference signals, and perform a repetition stepfor transmitting a plurality of second beams, to which differentsteering vectors are applied, to the user equipment throughcorresponding reference signals by considering feedback information thatincludes indexes of one or more beams received from the user equipment,and the repetition step may be repeated as much as a predeterminednumber of times.

Preferably, angles of the second beams may be determined in accordancewith angles of the one or more beams.

Preferably, the feedback information may further include at least anyone of signal strength on the one or more beams, channel qualityinformation (CQI), and precoding matrix indication (PMI).

Preferably, if the feedback information includes the PMI, the PMI may bedetermined on the basis of the first beams or the second beams, or maybe determined on the basis of one or beams selected by the userequipment through signal strength of the beams.

Preferably, if the feedback information includes signal strength on aplurality of beams, angles of the second beams may be determined at anunequal interval by considering signal strength on the plurality ofbeams.

Preferably, the first beams or the second beams may be generated usingantenna ports only having a predetermined interval, may be generatedusing antenna ports grouped per a predetermined number, or may begenerated by being grouped per a predetermined number.

Preferably, the reference signal may be a channel state informationreference signal (CSI-RS).

In still another aspect of the present invention, a method forperforming hierarchical beamforming in a wireless access systemcomprises allowing a user equipment to receive parameters including astep size for the hierarchical beamforming and the number of beams ateach step for the hierarchical beamforming from a base station; aninitial step for allowing the user equipment to transmit a plurality offirst beams, to which different steering vectors are applied, to thebase station through corresponding reference signals; and a repetitionstep for allowing the user equipment to transmit a plurality of secondbeams, to which different steering vectors are applied, to the basestation through corresponding reference signals by considering feedbackinformation that includes indexes of one or more beams received from thebase station, wherein the repetition step may be repeated as much as thestep size.

In further still another aspect of the present invention, a userequipment for performing hierarchical beamforming in a wireless accesssystem comprises a radio frequency (RF) unit configured to transmit andreceive a radio signal; and a processor, wherein the processor isconfigured to receive parameters including a step size for thehierarchical beamforming and the number of beams at each step for thehierarchical beamforming from a base station, perform an initial stepfor transmitting a plurality of first beams, to which different steeringvectors are applied, to the base station through corresponding referencesignals, and perform a repetition step for transmitting a plurality ofsecond beams, to which different steering vectors are applied, to thebase station through corresponding reference signals by consideringfeedback information that includes indexes of one or more beams receivedfrom the base station, and the repetition step may be repeated as muchas the step size.

Preferably, angles of the second beams may be determined in accordancewith angles of the one or more beams.

Preferably, the feedback information may further include at least anyone of signal strength on the one or more beams, channel qualityinformation (CQI), and precoding matrix indication (PMI).

Preferably, if the feedback information includes the PMI, the PMI may bedetermined on the basis of the first beams or the second beams, or maybe determined on the basis of one or beams selected by the base stationthrough signal strength of the beams.

Preferably, if the feedback information includes signal strength on aplurality of beams, angles of the second beams may be determined at anunequal interval by considering signal strength on the plurality ofbeams.

Preferably, the first beams or the second beams may be generated usingantenna ports only having a predetermined interval, may be generatedusing antenna ports grouped per a predetermined number, or may begenerated by being grouped per a predetermined number.

Preferably, the reference signal may be a channel state informationreference signal (CSI-RS).

Advantageous Effects

According to the embodiment of the present invention, beamforming may beperformed desirably between a user equipment and a base station in awireless access system, preferably a wireless access system thatsupports massive multi-input multi-output (MIMO).

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram illustrating physical channels used in a 3GPP LTEsystem and a general method for transmitting a signal using the physicalchannels;

FIG. 2 is a diagram illustrating a structure of a radio frame used in a3GPP LTE system;

FIG. 3 is a diagram illustrating an example of a resource grid of onedownlink slot;

FIG. 4 is a diagram illustrating a structure of a downlink subframe;

FIG. 5 is a diagram illustrating a structure of an uplink subframe;

FIG. 6 is a diagram illustrating a pattern where a common referencesignal (CRS) is arranged on a resource block if a normal cyclic prefixis used;

FIGS. 7 and 8 are diagrams illustrating patterns where a userequipment-specific reference signal (DM-RS) is arranged on a resourceblock if a normal cyclic prefix is used;

FIG. 9 is a diagram illustrating a pattern where CSI-RS based on CSI-RSconfiguration #0 is arranged on a resource block if a normal cyclicprefix is used;

FIG. 10 is a diagram illustrating a conventional beamforming operation;

FIG. 11 is a diagram illustrating a hierarchical beamforming methodaccording to one embodiment of the present invention;

FIGS. 12 to 14 are diagrams briefly illustrating a hierarchicalbeamforming operation according to one embodiment of the presentinvention;

FIG. 15 is a diagram illustrating a beam angle adaptation method basedon un-equal quantization according to the present invention;

FIGS. 16 to 18 are graphs illustrating a simulation result based on ahierarchical beamforming scheme according to the present invention; and

FIG. 19 is a block diagram illustrating a wireless communication systemaccording to one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings. It isto be understood that the detailed description, which will be disclosedalong with the accompanying drawings, is intended to describe theexemplary embodiments of the present invention, and is not intended todescribe a unique embodiment with which the present invention can becarried out. The following detailed description includes detailedmatters to provide full understanding of the present invention. However,it will be apparent to those skilled in the art that the presentinvention can be carried out without the detailed matters.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus.

In this specification, the embodiments of the present invention havebeen described based on the data transmission and reception between abase station and a user equipment. In this case, the base station meansa terminal node of a network, which performs direct communication withthe user equipment. Herein, a specific operation which has beendescribed as being performed by the base station may be performed by anupper node of the base station as the case may be. In other words, itwill be apparent that various operations performed for communicationwith the user equipment in the network which includes a plurality ofnetwork nodes along with the base station may be performed by the basestation or network nodes other than the base station. The base station(BS) may be replaced with terms such as a fixed station, Node B, eNode B(eNB), and an access point (AP). A relay may be replaced with terms suchas relay node (RN) and relay station (RS). Also, terminal may bereplaced with terms such as a user equipment (UE), a mobile station(MS), a mobile subscriber station (MSS), a subscriber station (SS), anadvanced mobile station (AMS), a wireless terminal (WT), a machine-typecommunication (MTC) device, a machine-to-machine (M2M) device, and adevice-to-device (D2D) device.

Specific terminologies used in the following description are provided toassist understanding of the present invention, and various modificationsmay be made in the specific terminologies within the range that they donot depart from technical spirits of the present invention.

The embodiments of the present invention may be supported by standarddocuments disclosed in at least one of wireless access systems, i.e.,IEEE 802.xx system, 3GPP system, 3GPP LTE system, and 3GPP2 system.Namely, among the embodiments of the present invention, steps or partswhich are not described to clarify the technical spirits of the presentinvention may be supported by the above standard documents. Also, allterminologies disclosed herein may be described by the above standarddocuments.

The following technology may be used for various wireless accesstechnologies such as CDMA (code division multiple access), FDMA(frequency division multiple access), TDMA (time division multipleaccess), OFDMA (orthogonal frequency division multiple access), andSC-FDMA (single carrier frequency division multiple access). The CDMAmay be implemented by the radio technology such as UTRA (universalterrestrial radio access) or CDMA2000. The TDMA may be implemented bythe radio technology such as global system for mobile communications(GSM)/general packet radio service (GPRS)/enhanced data rates for GSMevolution (EDGE). The OFDMA may be implemented by the radio technologysuch as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, andevolved UTRA (E-UTRA). The UTRA is a part of a universal mobiletelecommunications system (UMTS). A 3rd generation partnership projectlong term evolution (3GPP LTE) is a part of an evolved UMTS (E-UMTS)that uses E-UTRA, and adopts OFDMA in a downlink and SC-FDMA in anuplink. LTE-advanced (LTE-A) is an evolved version of the 3GPP LTE.

For clarification of the description, although the following embodimentswill be described based on the 3GPP LTE/LTE-A, it is to be understoodthat the technical spirits of the present invention are not limited tothe 3GPP LTE/LTE-A.

3GPP LTE/LTE-A System to which the Present Invention May be Applied

FIG. 1 is a diagram illustrating physical channels used in a 3GPP LTEsystem and a general method for transmitting a signal using the physicalchannels.

The user equipment, of which power is turned on, or which newly enters acell, performs initial cell search such as synchronizing with the basestation at step S101. To this end, the user equipment synchronizes withthe base station by receiving a primary synchronization channel (P-SCH)and a secondary synchronization channel (S-SCH) from the base station,and acquires information such as cell ID, etc.

Afterwards, the user equipment may acquire broadcast information withinthe cell by receiving a physical broadcast channel (PBCH) from the basestation. Meanwhile, the user equipment may identify a downlink channelstatus by receiving a downlink reference signal (DL RS) at the initialcell search step.

The user equipment which has finished the initial cell search mayacquire more detailed system information by receiving a physicaldownlink control channel (PDCCH) and a physical downlink shared channel(PDSCH) based on the PDCCH at step S102.

Afterwards, the user equipment may perform a random access procedure(RACH) such as steps S103 to S106 to complete access to the basestation. To this end, the user equipment may transmit a preamble througha physical random access channel (PRACH) (S103), and may receive aresponse message to the preamble through the PDCCH and the PDSCHcorresponding to the PDCCH (S104). In case of a contention based RACH,the user equipment may perform a contention resolution procedure such astransmission (S105) of additional physical random access channel andreception (S106) of the physical downlink control channel and thephysical downlink shared channel corresponding to the physical downlinkcontrol channel.

The user equipment which has performed the aforementioned steps mayreceive the physical downlink control channel (PDCCH)/physical downlinkshared channel (PDSCH) (S107) and transmit a physical uplink sharedchannel (PUSCH) and a physical uplink control channel (PUCCH) (S108), asa general procedure of transmitting uplink/downlink signals.

Control information transmitted from the user equipment to the basestation will be referred to as uplink control information (UCI). The UCIincludes HARQ ACK/NACK (Hybrid Automatic Repeat and reQuestAcknowledgement/Negative-ACK), SR (Scheduling Request), CQI (channelquality indicator), PMI (precoding matrix indicator), RI (rankindication) information, etc.

Although the UCI is generally transmitted through the PUCCH in the LTEsystem, it may be transmitted through the PUSCH if control informationand traffic data should be transmitted at the same time. Also, the userequipment may non-periodically transmit the UCI through the PUSCH inaccordance with request/command of the network.

FIG. 2 is a diagram illustrating a structure of a radio frame in a 3GPPLTE.

In a cellular OFDM wireless packet communication system, uplink/downlinkdata packet transmission is performed in a subframe unit, wherein onesubframe is defined by a given time interval that includes a pluralityof OFDM symbols. The 3GPP LTE standard supports a type 1 radio framestructure applicable to frequency division duplex (FDD) and a type 2radio frame structure applicable to time division duplex (TDD).

FIG. 2( a) is a diagram illustrating a structure of a type 1 radioframe. The downlink radio frame includes 10 subframes, each of whichincludes two slots in a time domain. A time required to transmit onesubframe will be referred to as a transmission time interval (TTI). Forexample, one subframe may have a length of 1 ms, and one slot may have alength of 0.5 ms. One slot includes a plurality of OFDM symbols in atime domain and a plurality of resource blocks (RB) in a frequencydomain. Since the 3GPP LTE system uses OFDM in a downlink, OFDM symbolsis intended to express one symbol interval. The OFDM symbols may bereferred to as one SC-FDMA symbol or symbol interval. The resource block(RB) as a resource allocation unit may include a plurality of continuoussubcarriers in one slot.

The number of OFDM symbols included in one slot may be varied dependingon configuration of a cyclic prefix (CP). Examples of the CP include anextended CP and a normal CP. For example, if the OFDM symbols areconfigured by the normal CP, the number of OFDM symbols included in oneslot may be 7. If the OFDM symbols are configured by the extended CP,since the length of one OFDM symbol is increased, the number of OFDMsymbols included in one slot is smaller than that of OFDM symbols incase of the normal CP. For example, in case of the extended CP, thenumber of OFDM symbols included in one slot may be 6. If a channel stateis unstable like the case where the user equipment moves at high speed,the extended CP may be used to reduce inter-symbol interference.

If the normal CP is used, since one slot includes seven OFDM symbols,one subframe includes 14 OFDM symbols. At this time, first maximum threeOFDM symbols of the subframe may be allocated to a physical downlinkcontrol channel (PDCCH), and the other OFDM symbols may be allocated toa physical downlink shared channel (PDSCH).

FIG. 2( b) is a diagram illustrating a structure of a type 2 radioframe. The type 2 radio frame includes two half frames, each of whichincludes five subframes, a downlink pilot time slot (DwPTS), a guardperiod (GP), and an uplink pilot time slot (UpPTS). One of the fivesubframes includes two slots. The DwPTS is used for initial cell search,synchronization or channel estimation at the user equipment. The UpPTSis used to synchronize channel estimation at the base station withuplink transmission of the user equipment. Also, the guard period is toremove interference occurring in the uplink due to multipath delay ofdownlink signals between the uplink and the downlink.

The aforementioned structure of the radio frame is only exemplary, andvarious modifications may be made in the number of subframes included inthe radio frame or the number of slots included in the subframe, or thenumber of symbols included in the slot.

FIG. 3 is a diagram illustrating a resource grid for a downlink slotused in an LTE system.

Referring to FIG. 3, one downlink slot includes a plurality of OFDMsymbols in a time domain. In this case, one downlink slot includes, butnot limited to, seven OFDM symbols, and one resource block (RB)includes, but not limited to, twelve subcarriers in a frequency domain.

Each element on the resource grid will be referred to as a resourceelement (RE). One resource block (RB) includes 12×7 resource elements.The number N^(DL) of resource blocks (RBs) included in the downlink slotdepends on a downlink transmission bandwidth. A structure of an uplinkslot may be the same as that of the downlink slot.

FIG. 4 is a diagram illustrating a structure of a downlink subframe.

Referring to FIG. 4, maximum three OFDM symbols located at the front ofthe first slot within one subframe correspond to a control region towhich control channels are allocated. The other OFDM symbols correspondto a data region to which a physical downlink shared channel (PDSCH) isallocated. Examples of the downlink control channel used in the 3GPP LTEinclude a PCFICH (Physical Control Format Indicator CHannel), a PDCCH(Physical Downlink Control CHannel), and a PHICH (Physical Hybrid ARQIndicator CHannel).

The PCFICH is transmitted from the first OFDM symbol of the subframe,and carries information on the number (that is, size of the controlregion) of OFDM symbols used for transmission of the control channelswithin the subframe. The PHICH is a response channel to uplinktransmission, and carries HARQ ACK/NACK(acknowledgement/negative-acknowledgement) signal. The controlinformation transmitted through the PDCCH will be referred to asdownlink control information (DCI). The downlink control information(DCI) includes uplink resource allocation information, downlink resourceallocation information, or uplink transmission (Tx) power controlcommand for a random user equipment group.

The PDCCH may carry resource allocation and transport format (that maybe referred to as downlink grant) of a downlink shared channel (DL-SCH),resource allocation information (that may be referred to as uplinkgrant) of an uplink shared channel (UL-SCH), paging information on apaging channel (PCH), system information on the DL-SCH, resourceallocation information of upper layer control message such as randomaccess response transmitted on the PDSCH, a set of transmission powercontrol commands of individual user equipments (UEs) within a randomuser equipment group, and activity indication information of voice overInternet protocol (VoIP). A plurality of PDCCHs may be transmittedwithin the control region, and the user equipment may monitor theplurality of PDCCHs. The PDCCH is configured by aggregation of one or aplurality of continuous control channel elements (CCEs). The CCE is alogic allocation unit used to provide the PDCCH with a coding rate basedon the status of a radio channel. The CCE corresponds to a plurality ofresource element groups (REGs). The format of the PDCCH and the numberof available bits of the PDCCH are determined depending on thecorrelation between the number of CCEs and the coding rate provided bythe CCEs.

The base station determines a PDCCH format depending on the DCI whichwill be transmitted to the user equipment, and attaches cyclicredundancy check (CRC) to the control information. The CRC is maskedwith a unique identifier (for example, radio network temporaryidentifier (RNTI)) depending on usage of the PDCCH or owner of thePDCCH. For example, if the PDCCH is for a specific user equipment, theCRC may be masked with cell-RNTI (C-RNTI) of the corresponding userequipment. If the PDCCH is for a paging message, the CRC may be maskedwith a paging indication identifier (for example, paging-RNTI (P-RNTI)).If the PDCCH is for system information (in more detail, systeminformation block (SIB)), the CRC may be masked with system informationRNTI (SI-RNTI). If the PDCCH is for a random access response, the CRCmay be masked with a random access RNTI (RA-RNTI) to indicate a randomaccess response which is a response to transmission of a random accesspreamble.

FIG. 5 is a diagram illustrating a structure of an uplink subframe.

Referring to FIG. 5, the uplink subframe may be divided into a controlregion and a data region in a frequency domain. A physical uplinkcontrol channel (PUCCH) which carries uplink control information isallocated to the control region. A physical uplink shared channel(PUSCH) which carries user data is allocated to the data region. Inorder to maintain single carrier features, one user equipment does nottransmit the PUCCH and the PUSCH at the same time. A resource block (RB)pair for the subframe is allocated to the PUCCH for one user equipment.Resource blocks (RBs) belonging to the RB pair reserve their respectivesubcarriers different from each other at each of two slots. The RB pairallocated to the PUCCH is subjected to frequency hopping at a slotboundary.

Downlink Reference Signal and Downlink Measurement

When a packet (or signal) is transmitted in the wireless communicationsystem, signal distortion may occur during transmission of the packetbecause the packet is transmitted through a radio channel. In order tonormally receive the distorted signal, a receiver should compensatedistortion of the received signal by using channel information. In orderto discover the channel information, it is required to transmit thesignal known by both a transmitter and the receiver and discover thechannel information using a distortion level of the signal when thesignal is transmitted through the channel. In this case, the signalknown by both the transmitter and the receiver will be referred to as apilot signal or a reference signal.

In case that the transmitter or the receiver of the wirelesscommunication system transmits and receives by using multiple antennasto increase capacity and improve communication throughput, a channelstate between each transmitter and each receiver should be known toreceive a normal signal. Accordingly, a separate reference signal shouldbe provided per transmitting antenna.

In the wireless communication system, the reference signal may bedivided into two types in accordance with its purpose. Examples of thereference signal include a reference signal used for acquisition ofchannel information and a reference signal used for data demodulation.Since the former reference signal is intended for acquisition of channelinformation on the downlink through the user equipment, it needs to betransmitted through a wideband. Also, the former reference signal shouldbe received even by a user equipment that does not receive downlink datafor a specific subframe. Also, this reference signal for acquisition ofchannel information may be used even for measurement of handover. Thelatter reference signal is transmitted from the base station togetherwith a corresponding resource when the base station transmits downlinkdata. In this case, the user equipment may perform channel estimation byreceiving the corresponding reference signal, whereby the user equipmentmay demodulate the data. This reference signal for data demodulationshould be transmitted to a region to which data are transmitted.

The 3GPP LTE system defines a common reference signal (CRS) shared byall the user equipments within a cell and a dedicated reference signal(DRS) for a specific user equipment only as downlink reference signals.The CRS is used for both acquisition of channel information and datademodulation, and may be referred to as a cell-specific referencesignal. The base station transmits the CRS per subframe through awideband. On the other hand, the DRS is used for data demodulation only,and may be transmitted through resource elements if data demodulation onthe PDSCH is required. The user equipment may receive the presence ofthe DRS through upper layer signaling. The DRS is useful only if thecorresponding PDSCH signal is mapped. The DRS may be referred to as auser equipment-specific reference signal (UE-specific RS) ordemodulation reference signal (DMRS).

The receiver (user equipment) may estimate the channel status from theCRS, and may feed an indicator related to channel quality, such as achannel quality indicator (CQI), a precoding matrix index (PMI), and/ora rank indicator (RI), back to the transmitter (base station) inaccordance with the estimated channel status. Alternatively, thereference signal related to feedback of the channel status information(CSI) such as CQI/PMI/RI may separately be referred to as CSI-RS. TheCSI-RS for channel measurement is designed for channel measurementmainly unlike the existing CRS used for channel measurement and datademodulation. In this way, since the CSI-RS is transmitted only toobtain channel state information, the base station transmits the CSI-RSfor all the antenna ports. Also, since the CSI-RS is transmitted todiscover downlink channel information, the CSI-RS is transmitted to afull band unlike the DRS.

The current 3GPP LTE system defines two types transmission schemes, thatis, an open-loop MIMO transmission scheme operated without channelinformation of the receiver and a closed-loop MIMO transmission schemeoperated based on channel information. In the closed-loop MIMOtransmission scheme, each of the transmitter and the receiver performsbeamforming on the basis of channel information, that is, channel stateinformation to obtain multiplexing gain of MIMO antenna. To acquire CSIfrom the user equipment, the base station commands the user equipment tofeed downlink CSI back by allocating a physical uplink control channel(PUCCH) or a physical uplink shared channel (PUSCH) to the userequipment.

The CSI is classified into three types of information, that is, a rankindicator (RI), a precoding matrix index (PMI), and a channel qualityindicator (CQI).

The RI represents rank information of a channel, and means the number ofstreams (or layers) that are received by the user equipment through thesame frequency-time resource. Also, since RI is determined dominantly bylong term fading of the channel, it is fed back from the user equipmentto the base station at a time period longer than that of the PMI and theCQI.

The PMI is a value obtained by reflecting spatial properties of achannel, and represents a precoding index of the base station, which ispreferred by the user equipment, on the basis of metric such as SINR(Signal to Interference plus Noise Ratio). In other words, the PMI isinformation on a precoding matrix used for transmission from thetransmitter. The precoding matrix fed back from the receiver isdetermined considering the number of layers indicated by RI. The PMI maybe fed back in case of closed-loop special multiplexing and large delayCDD transmission. In case of open-loop transmission, the transmitter mayselect the precoding matrix in accordance with a rule which ispreviously determined A procedure of selecting PMI for each rank in thereceiver is as follows. The receiver may calculate SINR, which ispreviously processed, for each PMI, convert the calculated SINR to sumcapacity, and select the best PMI on the basis of the sum capacity. Inother words, PMI calculation of the receiver may be regarded as aprocedure of discovering the best PMI on the basis of the sum capacity.The transmitter that has received the PMI fed back from the receiver mayuse the precoding matrix recommended by the receiver as it is, and mayinclude the precoding matrix recommended by the receiver in datatransmission scheduling allocation information to the receiver as anindicator of 1 bit. Alternatively, the transmitter may not use theprecoding matrix indicated by the PMI fed back from the receiver, as itis. In this case, the transmitter may explicitly include precodingmatrix information used for data transmission to the receiver inscheduling allocation information.

The CQI is a value indicating strength of a channel, and means receivedSNR that may be obtained when the base station uses the PMI. The userequipment reports CQI index, which indicates specific combination in aset that includes combinations of predetermined modulation scheme andcoding rates, to the base station.

Hereinafter, the downlink reference signal will be described in detail.

FIG. 6 is a diagram illustrating a pattern where a common referencesignal (CRS) is arranged on a resource block if a normal cyclic prefixis used.

R0 to R3 shown in FIG. 6 represent resource elements to which CRS forantenna ports 0 to 3 is mapped. In other words, Rp represents a resourceelement into which reference signal transmission on an antenna portindex p is mapped.

The CRS is defined in various formats depending on antenna configurationof the transmitter (base station). The 3GPP LTE system supports variousantenna configurations, and a downlink signal transmitter (base station)has three types of antenna configurations of single antenna, twotransmitting antennas and four transmitting antennas. If multipleantennas are supported, one of the antenna ports transmits a referencesignal to a designated resource element (RE) location in accordance witha reference signal pattern, and does not transmit any signal to aresource element location designated for another antenna port.

A location on a frequency domain may be shifted so as not to generatecollision of reference signals per cell, whereby channel estimationthroughput through the CRS may be enhanced. For example, in view of oneantenna, each reference signal may be located in the frequency domain atan interval of 6 subcarriers. Accordingly, at least five neighboringcells may locate the reference signals at different locations of thefrequency domain through shifting of subcarrier unit in the frequencydomain.

Also, as the downlink reference signal per cell may be multiplied by asequence (for example, Pseduo-random (PN), m-sequence, etc.) which ispreviously defined, signal interference of pilot symbols received by thereceiver from the neighboring cells may be reduced, whereby channelestimation throughput may be improved. PN sequence is applied in a unitof OFDM symbol within one subframe, and different PN sequences may beapplied depending on cell ID, subframe number, OFDM symbol location, andID of the user equipment.

Since the DM-RS is the reference signal for data demodulation, the DM-RSis located in a region which a downlink data channel is allocated, andis allocated to a location, to which the CRS is not allocated, in theregion where the downlink data channel is allocated. The user equipmentis signaled as to the presence of the DM-RS through an upper layer, thatis, as to whether downlink data channel transmission is based on CRS orDM-RS.

FIGS. 7 and 8 are diagrams illustrating patterns where a userequipment-specific reference signal (DM-RS) is arranged on a resourceblock if a normal cyclic prefix is used.

In the 3GGP LTE system, DM-RS for antenna ports p=5, p=7, p=8 or p=7, 8,. . . , ν+6 is defined. In this case, ν means the number of layers towhich the PDSCH is transmitted. The DM RS for different antenna portsmay be identified from one another by being located on differentfrequency resource (subcarriers) and/or different time resources (OFDMsymbols). DM-RS set (S) may be divided into S={7,8,11,13} andS={9,10,12,14}, and may be transmitted to one user equipment through anyone antenna port included in a specific antenna port set (S). The DM-RSfor the antenna port {7, 8, 11, 13} included in the DM-RS set 1 may bemapped into the same resource element, and may be multiplexed byorthogonal code. If the number of layers transmitted to the userequipment is small (for example, the number of transmitted layers is 1to 2), DRS pattern for antenna ports included in one set may be used.However, if a lot of layers are transmitted to the user equipment (forexample, the number of transmitted layers is 3 to 8), DRS pattern forantenna ports included in two sets may be used.

FIG. 7 illustrates a pattern of DM-RS transmitted through antenna port5, and FIG. 8 illustrates a pattern of DM-RS transmitted through antennaports 7 to 10. R5, R7 to R10 shown in FIGS. 7 and 8 respectivelyrepresent resource elements to which DM-RS for antenna ports 5, 7 to 10are mapped. In other words, Rp represents a resource element to whichreference signal transmission on the antenna port p is mapped.

In the system (for example, LTE-A system that supports 8 Tx antennas)having antenna configuration more extended than that of the system (forexample, LTE release 8 system that supports 4 Tx antennas) having theconventional antenna configuration, transmission of a new referencesignal for acquiring channel state information (CSI) is required. Sincethe aforementioned CRS is the reference signal for antenna ports 0 to 3,it is required that a new reference signal for acquiring a channel stateon the extended antenna ports should be designed additionally.

CSI-RS has been suggested for the purpose of channel measurement of thePDSCH separately from the CRS. Unlike the CRS, the CSI-RS may be definedby maximum 32 types of different configurations to reduce inter-cellinterference (ICI) in a multi-cell environment.

Configuration for the CSI-RS is varied depending on the number ofantenna ports of a cell, and CSI-RS defined by different configurationsif possible are transmitted between neighboring cells. Also, CSI-RSconfiguration is identified by a type (normal cyclic prefix or extendedcyclic prefix) of cyclic prefix, and may be divided into a case wherethe CSI-RS configuration is applied to both FS1 and FS2 in accordancewith a frame structure (FS) type and a case where the CSI-RSconfiguration is applied to FS2 only. Also, unlike the CRS, the CSI-RSsupports maximum 8 antenna ports (p=15, p=15, 16, p=15, . . . , 18 orp=15, . . . , 22), and is defined for Δf=15 kHz only.

FIG. 9 is a diagram illustrating a pattern where CSI-RS based on CSI-RSconfiguration #0 is arranged on a resource block if a normal cyclicprefix is used.

Referring to FIG. 9, a resource element to which the CSI-RS istransmitted is located on one resource block to which downlink data istransmitted. The CSI-RS for different antenna ports may be identifiedfrom one another by being located on different frequency resources(subcarriers) and/or different time resources (OFDM symbols). Also, theCSI-RS for different antenna ports located on the same time-frequencyresource may be identified from one another by orthogonal code.

In the example of FIG. 8, CSI-RS for antenna ports 15 and 16, CSI-RS forantenna ports 17 and 18, CSI-RS for antenna ports 19 and 20, and CSI-RSfor antenna ports 21 and 22 may be located on the same resource element,and may be multiplexed by orthogonal code.

Hereinafter, multiple CSI-RS configurations will be described.

Multiple CSI-RS configurations may be used within a single cell. Inother words, one (or 0) configuration that assumes non-zero transmissionpower for CSI-RS in the user equipment and a plurality of configurations(or 0) that assume zero transmission power in the user equipment may beused.

For each bit set to 1 in 16 bit bitmap ‘ZeroPowerCSI-RS’ configured byan upper layer, the user equipment assumes zero transmission power in aresource element corresponding to 4 CSI-RS columns of Table 1 and Table2 below. At this time, resource elements overlapped with the non-zerotransmission power CSI-RS resource element configured by the upper layerare excluded. The highest bit of the bitmap corresponds to the lowestCSI-RS configuration index, and next bits sequentially correspond toCSI-RS configuration index.

The CSI-RS may exist on a downlink slot only that satisfies n_(s) mod 2in Table 1 and Table 2 respectively based on normal cyclic prefix andextended cyclic prefix.

The user equipment assumes that the CSI-RS is not transmitted in case ofthe following cases:

-   -   special subframe in case of frame structure type 2;    -   subframe at which transmission of the CSI-RS collide with        transmission of synchronization signal, physical broadcast        channel (PBCH) or SystemInformationBlockType1 message; and    -   subframe at which transmission of a paging message is        configured.

The antenna port (S) may be identified by S={15}, S={15,16}, S={17,18},S={19,20} or S={21,22}. A resource element (k,l) used for CSI-RStransmission on a specific antenna port within one antenna port set isnot used for PDSCH transmission through another antenna port within thesame slot, and is not used for CSI-RS transmission for another antennaport within the corresponding antenna port set (S) within the same slot.

Table 1 illustrates a mapping relation of a resource element (k′,l′)based on CSI-RS configuration if normal cyclic prefix is used.

TABLE 1 Number of CSI reference signals configured CSI reference signal1 or 2 4 8 configuration (k′, l′) n_(s) mod 2 (k′, l′) n_(s) mod 2 (k′,l′) n_(s) mod 2 Frame 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 structure 1 (11, 2) 1 (11, 2)  1 (11, 2)  1 type 2 (9, 2) 1 (9, 2) 1 (9, 2) 1 1 and 2 3 (7,2) 1 (7, 2) 1 (7, 2) 1 4 (9, 5) 1 (9, 5) 1 (9, 5) 1 5 (8, 5) 0 (8, 5) 06 (10, 2)  1 (10, 2)  1 7 (8, 2) 1 (8, 2) 1 8 (6, 2) 1 (6, 2) 1 9 (8, 5)1 (8, 5) 1 10 (3, 5) 0 11 (2, 5) 0 12 (5, 2) 1 13 (4, 2) 1 14 (3, 2) 115 (2, 2) 1 16 (1, 2) 1 17 (0, 2) 1 18 (3, 5) 1 19 (2, 5) 1 Frame 20(11, 1)  1 (11, 1)  1 (11, 1)  1 structure 21 (9, 1) 1 (9, 1) 1 (9, 1) 1type 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 2 only 23 (10, 1)  1 (10, 1)  1 24(8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1) 1 27 (4, 1) 1 28 (3, 1)1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 1

Table 2 illustrates a mapping relation of a resource element (k′,l′)based on CSI-RS configuration if extended cyclic prefix is used.

TABLE 2 Number of CSI reference signals configured CSI reference signal1 or 2 4 8 configuration (k′, l′) n_(s) mod 2 (k′, l′) n_(s) mod 2 (k′,l′) n_(s) mod 2 Frame 0 (11, 4)  0 (11, 4)  0 (11, 4) 0 structure 1 (9,4) 0 (9, 4) 0  (9, 4) 0 type 2 (10, 4)  1 (10, 4)  1 (10, 4) 1 1 and 2 3(9, 4) 1 (9, 4) 1  (9, 4) 1 4 (5, 4) 0 (5, 4) 0 5 (3, 4) 0 (3, 4) 0 6(4, 4) 1 (4, 4) 1 7 (3, 4) 1 (3, 4) 1 8 (8, 4) 0 9 (6, 4) 0 10 (2, 4) 011 (0, 4) 0 12 (7, 4) 1 13 (6, 4) 1 14 (1, 4) 1 15 (0, 4) 1 Frame 16(11, 1)  1 (11, 1)  1 (11, 1) 1 structure 17 (10, 1)  1 (10, 1)  1(10, 1) 1 type 18 (9, 1) 1 (9, 1) 1  (9, 1) 1 2 only 19 (5, 1) 1 (5, 1)1 20 (4, 1) 1 (4, 1) 1 21 (3, 1) 1 (3, 1) 1 22 (8, l)  1 23 (7, 1) 1 24(6, 1) 1 25 (2, 1) 1 26 (1, 1) 1 27 (0, 1) 1

The CSI-RS may be transmitted at a specific subframe not every subframe.In more detail, the CSI-RS may be transmitted at a subframe, whichsatisfies the following Equation 1, with reference to CSI-RS subframeconfiguration as illustrated in Table 3 below.

TABLE 3 CSI-RS CSI-RS CSI-RS- periodicity T_(CSI-RS) subframe offsetΔ_(CSI-RS) SubframeConfig I_(CSI-RS) (subframes) (subframes) 0-4 5I_(CSI-RS)  5-14 10 I_(CSI-RS) - 5 15-34 20 I_(CSI-RS) - 15 35-74 40I_(CSI-RS) - 35  75-154 80 I_(CSI-RS) - 75

In Table 3, T_(CSI-RS) means a period for transmission of the CSI-RS,Δ_(CSI-RS) means an offset value, n_(f) means a system frame number, andn_(s) means a slot number. I_(CSI-RS) may be set individually perCSI-RS.

(10n _(f) +└n _(s)/2┘−Δ_(CSI-RS))mod T _(CSI-RS)=0  [Equation 1]

Also, the aforementioned CSI-RS may be signaled to the user equipment asCSI-RS config information element as illustrated in Table 4 below.

TABLE 4 -- ASN1START CSI-RS-Config-r10 ::= SEQUENCE { csi-RS-r10 CHOICE{ release NULL, setup SEQUENCE { antennaPortsCount-r10 ENUMERATED {an1,an2, an4, an8}, resourceConfig-r10 INTEGER (0..31), subframeConfig-r10INTEGER (0..154), p-C-r10 INTEGER (−8..15) } } OPTIONAL, -- Need ONzeroTxPowerCSI-RS-r10 CHOICE { release NULL, setup SEQUENCE {zeroTxPowerResourceConfigList-r10 BIT STRING (SIZE (16)),zeroTxPowerSubframeConfig-r10 INTEGER (0..154) } } OPTIONAL -- Need ON }-- ASN1STOP

In Table 4, ‘antennaPortsCount-r10’ represents the number (selection of1, 2, 4 and 8) of antennas through which the CSI-RS is transmitted,‘resourceConfig-r10’ represents RE located within one RB on thetime-resource frequency, and ‘subframeConfig-r10’ represents a subframeat which CSI-RS EPRE value for PDSCH EPRE is transmitted. Additionally,the base station transfers information on zero power CSI-RS.

In CSI-RS configuration, ‘resourceConfig-r10’ represents a location towhich the CSI-RS is transmitted. This indicates exact symbol andsubcarrier location within one resource block in accordance with CSI-RSconfiguration number (see Table 1 or Table 2) expressed as numbers of 0to 31.

Table 5 illustrates description of a CSI-RS configuration field.

CSI-RS-Config field descriptions antennaPortsCount Parameter indicatingthe number of antenna ports used for CSI-RS transmission. Antenna 1corresponds to 1, antenna 2 corresponds to 2, and the others correspondlikewise. p-C P_(c)parameter resourceConfig parameter indicating CSI-RSconfiguration subframeConfig I_(CSI-RS)parameterzeroTxPowerResourceConfigList ZeroPowerCSI-RS parameterzeroTxPowerSubframeConfig I_(CSI-RS)parameter

Hierarchical Beamforming

Massive MIMO (Multi-Input Multi-Output) system may maximize beam gain byusing a lot of antennas and remove effect of intra-cell interference andnoise. In this massive MIMO system, a transmission scheme may be varieddepending on a duplex system such as TDD (Time Division Duplex) and FDD(Frequency Division Duplex).

The TDD system means that a downlink and an uplink use the samefrequency band and are identified from each other by time. Accordingly,if a coherence time of a radio channel is long, that is, if Dopplereffect is small, it may be assumed that radio channel features of thedownlink and the uplink are the same as each other. This may be referredto as reciprocity. Accordingly, the base station may perform channelestimation by using reference signals (RS) of the user equipments, whichare transmitted to the uplink, and may transmit a downlink signal byusing channel information estimated during downlink transmission. Inother words, since the base station does not need to transmit a separatedownlink reference signal to acquire downlink channel information, gainmay be obtained in view of resource overhead, and acts as great gain inthe massive MIMO system that uses a lot of antennas. Also, in view ofbeamforming which is a main purpose of the massive MIMO system, thetransmitter (for example, base station) may calculate a beamformingvector by using a channel or signal transmitted from the receiver (forexample, user equipment) on the basis of reciprocity in the TDD systemas described above. The beamforming vector means that weight valuesapplied to each antenna are configured as a vector. For example, if thebeamforming vector is w[w₁ w₂ . . . w_(N)]^(T), the transmitted signal smay be multiplied by w_(k) and then transmitted at the kth antenna.However, in case of the TDD system, a gap for a transition time betweenthe downlink and the uplink on the frame structure, that is, a gap for atransition guard time should be considered by considering round tripdelay. In other words, if cell coverage is greater, the transition guardtime is increased, which may act as throughput deterioration. For thisreason, the TDD system is accompanied with restrictions in cell coverageas compared with the FDD system. Also, the TDD system should considerthe same downlink/uplink (DL/UL) configuration between neighboring basestations to control interference between the neighboring base stations,and accompanies restrictions that uplink/downlink transmissionsynchronization between the base stations should be performed. Theduplex system of the massive MIMO may be considered even in the FDDsystem due to such a problem of the TDD system.

The FDD system is the system that the downlink uses frequency differentfrom that of the uplink. Accordingly, the base station cannot usechannel information estimated using the reference signals (RS) of theuser equipment, which are transmitted to the uplink during downlinktransmission, unlike the TDD system. In other words, since features ofchannel reciprocity cannot be used in the FDD system, another solutionshould be considered. Accordingly, in case of the FDD system, the basestation should transmit a reference signal to acquire channelinformation on the downlink and receive channel information fed backfrom the user equipment unlike the TDD system. In other words, the basestation provides a reference signal or pilot, which may estimate achannel of each antenna of the transmitter (for example, base station),and the receiver (for example, user equipment) reports channel stateinformation to the base station on the basis of the channel estimatedusing the reference signal. Hereinafter, the conventional beamformingoperation will be described with reference to FIG. 10.

FIG. 10 is a diagram illustrating the conventional beamformingoperation.

Referring to FIG. 10, if the conventional beamforming (CBF) is used, thebase station transmits a total of M reference signals to provide theuser equipment with M number of beam patterns. In the example of FIG.10, M=16. The user equipment reports beam of #2 from M number ofreference signals to the base station, and the base station transmits adownlink signal to the corresponding user equipment by performingprecoding corresponding to #2 during downlink transmission.

In the IEEE 802.16m or LTE/LTE-A system, the user equipment (that is,receiver) selects a proper beamforming vector (or precodingmatrix/vector) from a codebook corresponding to the number of antennasof the base station (that is, transmitter) and reports index of theselected result to the base station. In this codebook based beamforming(or precoding), the amount of information transmitted from the userequipment to the base station depends on a size of the codebook.Generally, since suboptimal performance as compared with optimizedperformance may be obtained in the 2, 4 or 8 Tx system even though acodebook size of 6 bits or less is used, the 2, 4, or 8 Tx system may beused preferably in a commercial system. Also, as a method for acquiringbeamforming gain in the FDD system, in addition to codebook basedbeamforming, a channel matrix or corresponding covariance matrix may bequantized and then notified to the base station, or an analog valuewhich is not quantized may be transmitted to the base station.

However, since a lot of antennas are considered in the massive MIMOsystem, feedback overhead to be transmitted by the user equipment aswell as reference signal overhead should be considered. If the FDDsystem assumes that the number of antennas of the base station is 100and all the antennas are respectively used for beamforming, the numberof resource elements (RE) to be used by the base station to transmit thereference signal is 100 or more. At this time, the resource elementmeans a resource that may be used in a code domain as well as time andfrequency domains. For example, in the LTE system, 8 (in case of singleantenna) resource elements, 16 (in case of 2 antennas) resourceelements, and 24 (in case of 4 antennas) resource elements are used totransmit the CRS within one resource block (RB), and 8 (in case of 8antennas) resource elements are used to transmit the CSI-RS. The problemoccurring as the number of antennas is increased may act as overhead infeedback information of the user equipment and codebook design as wellas reference signal overhead. For example, codebook design should beperformed in accordance with the number of antennas transmitted from thebase station. This may cause restrictions in the number of transmittingantennas from the base station or a lot of codebook types. Also, inorder to use codebook based closed loop MIMO, the number of dimensionsto be expressed by the codebook corresponding to the number ofcorresponding antennas is increased as the number of antennas isincreased, whereby the codebook size is increased proportionally.Accordingly, the user equipment should perform many operations tocalculate suitable PMI within the codebook, and the amount ofinformation to be fed back is increased in accordance with increase ofthe codebook types and codebook size.

The aforementioned conventional procedure of acquiring channelinformation on the downlink is not suitable in the massive MIMO systemthat considers a lot of antennas. Accordingly, a method for using themassive MIMO system in the FDD system, that is, a method for maintainingsystem overhead and complexity for supporting Tx beamforming gain withina reasonable range while sufficiently acquiring Tx beamforming gain if amassive antenna is installed in the transmitter in the FDD system willbe suggested.

In the present invention, random beamforming may be considered to reducereference signal overhead or feedback overhead. Random beamforming meansa scheme that a plurality of beam patterns are transmitted randomly inthe form of open loop or using limited information only without channelinformation received from the user equipment when beamforming isperformed, whereby signal-to-noise ratio (SNR) of the user equipment maybe increased. Various open loop beamforming techniques or varioustechniques for adaptation of random features of beam to informationreceived from the user equipment may be included in random beamformingHereinafter, the technique suggested in the present invention is basedon that beam pattern is hierarchically designed when random beamformingis performed and the user equipment feeds beam index back. This may bereferred to as hierarchical beamforming (HBF) or hierarchical beamselection (HBS). If the suggested technique is applied to the massiveMIMO system, the reference signal per antenna is not required to betransmitted, and the user equipment has only to feed its preferred beamindex only back without feeding channel information on each antennaback, whereby reference signal overhand and feedback overhead may bereduced.

FIG. 11 is a diagram illustrating a hierarchical beamforming methodaccording to one embodiment of the present invention.

Referring to FIG. 11, the transmitter transmits M number of beams to thereceiver by performing random beamforming (S1101). At this time, thetransmitter may transmit a beam pattern to the receiver by using M(M<=the number of antennas of the transmitter) number of referencesignals. In other words, the M number of beams may respectivelycorrespond to the reference signals. Also, the beam may be mapped intoeach antenna port and then transmitted in the form of the referencesignal.

The receiver measures M number of beams transmitted from thetransmitter, selects at least one or more N number of beams and thentransmits information on the selected beam to the transmitter (S1103).In this case, the information on the beam may include at least one ofindex (or reference signal index, antenna port index) of the selectedbeam, signal strength of the selected beam, channel state information(for example, CSI, CQI, PMI, RI, RSRP, etc.) of the selected beam, andsignal quality when PMI is used. The steps S1101 and S1103 may bereferred to as the first step or the initial step of the hierarchicalbeamforming method according to the present invention. The informationfed back from the user equipment at the first step or the initial stepmay be referred to as first feedback information.

Subsequently, the transmitter transmits M₁ number of beams to thereceiver by considering information on N number of beams received fromthe receiver at the first step (S1105). In this case, M₁ number of beams(for example, beam angle, etc.) may be determined by N number of beamsselected by the receiver at the previous step (that is, first step). Thereceiver selects at least one or more N₁ number of beams by measuring M₁number of beams transmitted from the transmitter and then transmitsinformation on the selected beam to the transmitter (S1107). Likewise,the information on the beam may include at least one of index (orreference signal index, antenna port index) of the selected beam, signalstrength of the selected beam, and channel state information (forexample, CSI, CQI, PMI, etc.) of the selected beam. The steps S1105 andS1107 may be referred to as the second step of the hierarchicalbeamforming method according to the present invention. Alternatively,the respective steps S1105 to S1111 after the aforementioned initialstep may be referred to as a repetition step, and the information fedback from the user equipment at the repetition step may be referred toas second feedback information.

Afterwards, as each step is repeated, the transmitter transmitsM_((j−1)) number of beams to the receiver by considering information onthe beam received at a previous J−1th step (s1109), and the receiverselects at least one or more N_((j−1)) number of beams by measuringM_((j−1)) number of beams transmitted from the transmitter, and thentransmits information on the selected beam to the transmitter (S1111).At this time, since hierarchical depth or step size that means thenumber of repetition times of each step is previously determined, thebase station and the user equipment may mutually know the hierarchicaldepth or step size, and the base station may notify the user equipmentof the hierarchical depth or step size through upper layer signaling.

The respective steps for the aforementioned hierarchical beamformingoperation may be configured to be performed periodically. Also, the beamprovided by the transmitter per step may be designed independently, andbehavior of the receiver at each step may be performed independently.

FIG. 12 is a diagram briefly illustrating a hierarchical beamformingoperation according to one embodiment of the present invention.

FIG. 12 illustrates that the transmitter transmits beams to the receiverthrough two steps and the receiver selects beam at each step andnotifies the transmitter of information on the selection beam. At thefirst step, the transmitter transmits M number of beams (M=4 in FIG.12), and the receiver may transmit information (for example, beam index,signal strength, channel state information (CQI), etc.) on one of Mnumber of beams to the transmitter. At the second step, the transmitterconfigures M₁ (M=M₁ or M≠M₁) number of beams on the basis of theinformation on the beam transmitted from the receiver and provides thereceiver with the configured beams. The receiver retransmits informationon one of the configured beams to the transmitter. In FIG. 12, thereceiver selects one beam in the same manner as the conventionalbeamforming scheme and reports information on the selected beam to thetransmitter. However, the receiver may select a plurality of beams andreport information on the selected beams to the transmitter, or mayreport different kinds of information per step to the transmitter. Thiswill be described with reference to FIG. 13.

FIG. 13 is a diagram briefly illustrating a hierarchical beamformingoperation according to another embodiment of the present invention.

Referring to FIG. 13, at the first step, the transmitter provides Mnumber of beams (M=4 in FIG. 13), and the receiver transmits at leastone of information (for example, beam index, signal strength, channelstate information (CQI), etc.) on N₁ number of beams of M number ofbeams to the transmitter. At the second step, the transmitter providesM₁ (M=M₁ or M≠M₁) number of beams on the basis of the information on thebeam transmitted from the receiver. The receiver transmits information(for example, beam index and/or PMI) on N₂ number of beams of M₁ numberof beams to the transmitter. PMI may be selected from N₂ Tx codebook. Atthis time, it is preferable that a value of N₂ is 2 and the receiverselects N₂ number of beams having the greatest signal strength. However,although PMI is transmitted as the information on the beam at the secondstep in FIG. 13, PMI may be included in the information on N₁ number ofbeams transmitted from the receiver at the first step. In this case, PMImay be selected from N₁ Tx codebook. Also, a preferable value of N₁ maybe 2, and it is preferable that the receiver selects N₁ number of beamshaving the greatest signal strength.

In an environment where line of sight (LOS) is dominant, it is likelythat N₁ number of beams (first step) or N₂ number of beams (second step)selected by the receiver have continuous indexes. In this case, when thereceiver reports the indexes on N₁ number of beams (first step) or N₂number of beams (second step) to the transmitter, the receiver maytransmit only the index (for example, the lowest index or the highestindex of N₁ number of beams or N₂ number of beams) to the transmitter.

In this way, as the hierarchical beamforming scheme is used, referencesignal overhead may be minimized as compared with the aforementionedConventional BeamForming (CBF). Although a total of 16 reference signalsshould be used to provide 16 beams in the example (M=16) of FIG. 10, atotal of 8 reference signals are used to provide the same beam qualityas that of FIG. 10 in the example of FIG. 12 or FIG. 13 in such a mannerthat 4 reference signals are used to provide 4 beams at the first stepand 4 beams are transmitted within a beam preferred by the userequipment at the second step (it is assumed that the same beams as thoseof the first step are used). In other words, the suggested technique mayreduce overhead by using the reference signals equivalent to M (thenumber of beams in CBF)/J (step size in HBF) while providing the samebeam quality as that of the CBF.

Also, the hierarchical beamforming scheme may provide beam qualityhigher than that of a case where the same reference signal overhead asthat of the CBF occurs. This will be described with reference to FIG.14.

FIG. 14 is other diagram briefly illustrating a hierarchical beamformingoperation according to one embodiment of the present invention.

Referring to FIG. 14, at the first step, 16 beam patterns aretransmitted using 16 (M=16) reference signals, and 16 beam patterns forthe beam preferred by the user equipment are transmitted at the secondstep, whereby more adaptive beamforming than the conventionalbeamforming may be performed.

The hierarchical beamforming scheme according to the present inventionmay be applied to both the downlink and the uplink. Hereinafter, forconvenience of description, the downlink and the uplink will bedescribed respectively.

Downlink

The hierarchical beamforming scheme according to the present inventionwill be described in more detail per step. Hereinafter, it is assumedthat the number of antennas of the base station is N and hierarchicaldept or step size is J.

1) First Step

The base station may transmit m (m<=N) number of beam patternscell-specifically by using m number of reference signals. For example,the conventional LTE/LTE-A system, since different reference signals aretransmitted per antenna port, if the base station uses 100 antennas, 100reference signals should be generated. However, according to the presentinvention, different reference signals per beam pattern may betransmitted. In other words, even though the base station uses 100antennas, reference signals equivalent to the number of beam patternsless than 100 may be transmitted. Finally, each beam pattern may bereferred to as a precoded reference signal (for example, CSI-RS). Forexample, if the present invention is applied to the LTE system, the basestation may transmit beam patterns to the user equipment by using mnumber of CSI-RS precoded to have directionality which is equallyspread.

At this time, to generate m number of beam patterns, a steering vectorexpressed by the following Equation 2 may be used for each referencesignal.

$\begin{matrix}{{a_{1} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & ^{{- j}\; {kd}\; \sin \; \theta_{1}^{1}} & \ldots & ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{1}^{1}}\end{bmatrix}}}{a_{2} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & ^{{- j}\; {kd}\; \sin \; \theta_{2}^{1}} & \ldots & ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{2}^{1}}\end{bmatrix}}}\vdots {a_{m} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & ^{{- j}\; {kd}\; \sin \; \theta_{m}^{1}} & \ldots & ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{m}^{1}}\end{bmatrix}}}} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Referring to Equation 2, k is 2π/λ, λ means a wavelength of transmittingfrequency, and d means a distance between antennas. θ_(m) ^(j) means anangle for the mth beam pattern at the jth step. β is a constant valuefor normalizing overall power of a signal transmitted to an antenna andmeans the square of sum (Euclidean norm value) of absolute values ofelements of the steering vector.

At this time, a beam width is determined due to the number of antennasused to generate a beam pattern. In other words, if a lot of antennasare used, a beam pattern having a narrower width may be generated.Accordingly, to provide a wider beam width, steering vectors asexpressed by the following Equations 3 to 5 may be used. Since the firststep is that the base station searches for directionality towards theuser equipment by transmitting a beam pattern to the user equipmentequally for all the directions, the base station may search fordirectionality towards all the user equipments, which belong to cellcoverage, by using the beam pattern having a wider width.

$\begin{matrix}{{a_{1} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & 0 & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{1}^{1}} & 0 & \ldots & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{1}^{1}} & 0\end{bmatrix}}}{a_{2} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & 0 & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{2}^{1}} & 0 & \ldots & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{2}^{1}} & 0\end{bmatrix}}}\vdots {a_{m} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & 0 & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{m}^{1}} & 0 & \ldots & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{m}^{1}} & 0\end{bmatrix}}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

Referring to the Equation 3, a value of 0 is applied to even numberedantennas of the steering vector, whereby the corresponding antennas donot transmit a signal. Since the beam patterns are generated using onlyhalf (N/2) of a total of antennas, a wider beam width may be provided.For example, as compared with the Equation 2, although an angle of eachbeam pattern which is generated may be the same as that generated by theEquation 2, the beam patterns are generated using less antennas, wherebyeach beam width may become wider. Also, although the beam patterns aregenerated using half (N/2) of a total of antennas in the Equation 3, thebeam patterns may be generated using the number of antennas (forexample, ¼, ⅛, etc of a total of antennas) different from the half of atotal of antennas. In this case, an interval of antennas used forgeneration of the beam patterns may be maintained uniformly to providethe same beam width per angle of each beam pattern. For example, if theantennas corresponding to ¼ of a total of antennas are used, theantennas having 4 intervals may be used to generate the beam patterns.

Alternatively, a given number of antennas may be grouped to configurethe steering vector as expressed by the following Equation 4.

                                     [Equation  4]$a_{1} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & 1 & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{1}^{1}} & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{1}^{1}} & \ldots & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{1}^{1}} & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{1}^{1}}\end{bmatrix}}$$a_{2} = {\frac{1}{\sqrt{\beta}}\left\lbrack \begin{matrix}1 & 1 & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{2}^{1}} & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{2}^{1}} & \ldots & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{2}^{1}} & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{2}^{1}}\end{matrix} \right\rbrack}$ ⋮$a_{m} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}1 & 1 & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{m}^{1}} & ^{{- j}\; k\; 2\; d\; \sin \; \theta_{m}^{1}} & \ldots & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{m}^{1}} & ^{{- j}\; {k{({N - 2})}}d\; \sin \; \theta_{m}^{1}}\end{bmatrix}}$

Referring to the Equation 4, as the same value is applied to theantennas which belong to the same group, the same signal is transmittedto each group, whereby a beam width wider than that of a case wheredifferent values equivalent to a total of antennas are applied to thecorresponding antennas may be provided. For example, as compared withthe Equation 2, although an angle of each beam pattern which isgenerated may be the same as that generated by the Equation 2, the beampatterns are generated using less antennas, whereby each beam width maybecome wider. Although two antennas belong to one group in the Equation4, the number of antennas constituting one group may be varied (forexample, 3 antennas, 4 antennas, etc.).

Alternatively, a certain number of beam patterns may be grouped, wherebythe grouped beam patterns may be transmitted as one reference signal.The steering vector corresponding to a case where two beam patterns aregrouped may be expressed by the following Equation 5.

                                     [Equation  5]$a_{1} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}2 & {^{{- j}\; {kd}\; \sin \; \theta_{1}^{1}} + ^{{- j}\; {kd}\; \sin \; \theta_{2}^{1}}} & \ldots & {^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{1}^{1}} + ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{2}^{1}}}\end{bmatrix}}$ $a_{2} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}2 & {^{{- j}\; {kd}\; \sin \; \theta_{3}^{1}} + ^{{- j}\; {kd}\; \sin \; \theta_{4}^{1}}} & \ldots & {^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{3}^{1}} + ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{4}^{1}}}\end{bmatrix}}$ ⋮$a_{\frac{m}{2}} = {\frac{1}{\sqrt{\beta}}\begin{bmatrix}2 & {^{{- j}\; {kd}\; \sin \; \theta_{m - 1}^{1}} + ^{{- j}\; {kd}\; \sin \; \theta_{m}^{1}}} & \ldots & {^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{m - 1}^{1}} + ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{m}^{1}}}\end{bmatrix}}$

Referring to the Equation 5, since two beam patterns having theirrespective angles different from each other are grouped to generate awider beam pattern, a wider beam width may be provided. For example,since a beam pattern having an angle of 30 degrees and a beam patternhaving an angle of 60 degrees are grouped to generate one beam pattern,the grouped beam pattern may have a beam pattern type having an anglebetween 30 degrees and 60 degrees. For example, as compared with theEquation 2, the number of reference signals which are generated may behalf of the reference signals generated in the Equation 2, and each beampattern generated by the Equation 5 may have a beam width correspondingto two beam widths generated by the Equation 2.

The user equipment that has received the beam patterns from the basestation feeds index of its preferred beam among the beam patternstransmitted from the base station back to the base station. At thistime, the index of the beam reported by the user equipment may be morethan 1. As described above, in order to obtain the index of the beampreferred by the user equipment, the user equipment may use receivedpower of the reference signals each of which has a beam pattern. Thismay be implemented even with less complexity.

Also, the user equipment may report signal strength of the referencesignal corresponding to each index or channel quality information (forexample, CSI, CQI, PMI, etc.) to the base station together with theindexes of the beams. At this time, if the user equipment reports signalstrength or channel quality information of each beam to the base stationwhile reporting the plurality of beam indexes, although signal strengthor channel quality information, which is reported for each beam, may bereported as an absolute value, it may be reported as a relative value(for example, relative difference value or relative ratio) between theplurality of beams which are reported. For example, a relative ratio ofreceived power between the plurality of beams which are reported ispreviously determined as a table as illustrated in Table 1 below, andthe user equipment may reduce the amount of information which is fedback, by reporting a bitmap indicating the relative ratio of thereceived power in the table to the base station.

Table 6 illustrates a relative ratio between the beams which arereported to the base station and a bitmap based on the relative ratio.

TABLE 6 Ratio Reporting bitmap 1:1   00 1:0.7 01 1:0.5 10 1:0.3 11

Referring to Table 6, the user equipment reports two beam indexes to thebase station, and Table 6 illustrates relative ratios of received powerbetween two beams and bitmaps corresponding to the relative ratios.Although Table 6 illustrates the relative values between two beams andthe bitmaps based on the relative values, relative values among beamsmore than 2 or more bits may be determined as a table. Also, if arelative value between two beams is more subdivided (for example, 8ratios), more bits may be used.

Also, as described above, when the user equipment feeds its preferredbeam index back to the base station, the user equipment may report PMIto the base station together with the beam index. At this time, PMImeans index which is previously determined and indicates a precodingmatrix existing within a codebook (for example, see Table 2) known bythe base station and the user equipment. Hereinafter, it is assumed thatthe user equipment selects two beam indexes and feeds the selected beamindexes back to the base station.

The user equipment may calculate an optimized precoding matrix by usingthe following Equation 6 and report PMI on the calculated precodingmatrix to the base station.

$\begin{matrix}{\left\{ {W,H} \right\} = {\arg {\max\limits_{\underset{\overset{\sim}{H} \in T}{\overset{\sim}{W} \in C}}{{\overset{\sim}{H}\overset{\sim}{W}}}_{F}^{2}}}} & \left\lbrack {{Equation}\mspace{14mu} 6} \right\rbrack\end{matrix}$

Table 7 illustrates a codebook when the number of reference signals is2.

TABLE 7 Codebook Number of layers ν index 1 2 0$\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\1\end{bmatrix}$ $\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}$ 1 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- 1}\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\1 & {- 1}\end{bmatrix}$ 2 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\j\end{bmatrix}$ $\frac{1}{2}\begin{bmatrix}1 & 1 \\j & {- j}\end{bmatrix}$ 3 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 \\{- j}\end{bmatrix}$ —

Referring to the Equation 6 and Table 7, {tilde over (W)} means one ofprecoding matrixes existing in a codebook C={W₀, W₁, W₂, W₃}, and {tildeover (H)} means a channel matrix (T=Channel response of {a_(i−1),a_(i)}) for two random beam patterns {a_(i−1), a_(i)} of beam patternsA={a₁, a₂, . . . , a_(m)} transmitted to the user equipment.

In this case, the user equipment may perform full search for itspreferred beam index and PMI by using the Equation 6 and then report thebeam index and PMI to the base station. In other words, the channelmatrix {tilde over (H)} for two random beam patterns of the full beampatterns transmitted to the user equipment and a random precoding vector{tilde over (W)} of the precoding vectors included in the codebookillustrated in Table 7 may be applied to the Equation 6, whereby theoptimized beam index and precoding matrix (that is, PMI), which have thegreatest channel size, may be obtained.

Alternatively, the user equipment may obtain PMI only based on itspreferred beam index by using the Equation 6 (hereinafter, referred toas partial search). In other words, the channel matrix {tilde over (H)}for two beam patterns (for example, two beam patterns having thegreatest received power) selected by the user equipment and a randomprecoding vector {tilde over (W)} of the precoding vectors included inthe codebook illustrated in Table 7 may be applied to the Equation 6,whereby the optimized precoding matrix (that is, PMI) having thegreatest channel size may only be obtained.

In this way, if the user equipment reports PMI to the base stationtogether with its preferred beam index, the base station applies theprecoding matrix based on the PMI received from the user equipment tothe beam pattern which is transmitted at next step. In other words, thebase station multiples the steering vector applied at next step by theprecoding matrix reported by the user equipment to generate the beampattern of next step and then transmits the generated beam pattern tothe user equipment.

2) Second Step

The base station that has received the beam index from the userequipment generates second m′ number of beam patterns UE-specifically byusing the beam index fed back from the user equipment. At this time, thenumber of the beam patterns generated at the second step may be the sameas that of the beam patterns generated at the first step (m=m′), or maybe different from that of the beam patterns generated at the first step(mm′ or m/2≠m′ of Equation 5).

Also, m′ number of beam angles generated at the second step may beobtained using the beam index reported by the user equipment at thefirst step. In other words, m′ number beam patterns having specificdirectionality are generated considering directionality of the beamindex reported by the user equipment at the first step. For example, ifthe angle preferred by the user equipment is θ₂ ¹, the m′ number of beamangles generated at the second step have values of θ₁ ¹<θ_(1, . . . m′)²<θ₃ ¹. At this time, the steering vector expressed by the followingEquation 7 may be used for each reference signal to generate the m′number of beam patterns.

$\begin{matrix}{{a_{1} = {\frac{1}{\sqrt{N}}\begin{bmatrix}1 & ^{{- j}\; {kd}\; \sin \; \theta_{1}^{2}} & \ldots & ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{1}^{2}}\end{bmatrix}}}{a_{2} = {\frac{1}{\sqrt{N}}\begin{bmatrix}1 & ^{{- j}\; {kd}\; \sin \; \theta_{2}^{2}} & \ldots & ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{2}^{2}}\end{bmatrix}}}\vdots {a_{m^{\prime}} = {\frac{1}{\sqrt{N}}\begin{bmatrix}1 & ^{{- j}\; {kd}\; \sin \; \theta_{m^{\prime}}^{2}} & \ldots & ^{{- j}\; {k{({N - 1})}}d\; \sin \; \theta_{m^{\prime}}^{2}}\end{bmatrix}}}} & \left\lbrack {{Equation}\mspace{14mu} 7} \right\rbrack\end{matrix}$

Referring to the Equation 7, k is 2π/λ, λ means a wavelength of atransmitting frequency, and d means a distance between antennas. θ_(m)^(j) means an angle for the mth beam pattern at the jth step. β is aconstant value for normalizing overall power of a signal transmitted toan antenna and means the square of sum (Euclidean norm value) ofabsolute values of elements of the steering vector.

Also, various steering vectors described at the first step may be usedas the steering vector at the second step. In other words, the steeringvector expressed by the Equations 3 to 5 may be reused. For example, ifthe Equation 3 is used, the beam patterns may be generated using theantennas spaced apart from one another at a certain interval among thefull antennas. If the Equation 4 is used, a certain number of antennasmay be grouped to generate the beam patterns. If the Equation 5 is used,a certain number of beam patterns may be grouped to generate one beampattern.

If the beam indexes fed back from the user equipment are two or more andabsolute information or relative information on signal strength of eachbeam is reported to the base station, the angle value for generating thebeam pattern of the second step in the base station may be determinedadaptively by un-equal quantization.

FIG. 15 is a diagram illustrating a beam angle adaptation method basedon un-equal quantization according to the present invention.

In FIG. 15, it is assumed that the beam indexes reported by the userequipment at the first step are a2 and a3, a beam angle of a2 is θ₂ ¹,and a beam angle of a3 is θ₃ ¹.

If the user equipment reports the beam indexes a2, a3 only to the basestation, the base station may determine an angle value for generatingthe beam patterns of the second step at an equal interval by usingequal/linear quantization 1510. In other words, an angle of the beamhaving index of a2+(a3−a2)/3 may be determined as θ₂ ¹+(θ₃ ¹−θ₂ ¹)/3,and an angle of the beam having index of a2+2(a3−a2)/3 may be determinedas θ₂ ¹+2(θ₃ ¹−θ₂ ¹)/3.

On the other hand, if the user equipment reports strength of each beamtogether with the beam indexes a2, a3 to the base station (it is assumedthat ‘a2 beam strength<a3 beam strength’), the base station maydetermine an angle value for generating the beam patterns of the secondstep at an unequal interval by using unequal/nonlinear quantization1520. In other words, if beam strength of the first step becomes closeto the angle (a3 in FIG. 15) of a relatively great beam, a beam angleinterval θ₁ ²−θ₂ ¹ may be dense. If beam strength of the first stepbecomes close to the angle (a2 in FIG. 15) of a relatively small beam, abeam angle interval θ₃ ¹−θ₂ ² may be wide. In this case, for applicationof the unequal quantization 1520, an angle of a slope of the unequalquantization 1520 may be determined as a relative value (for example,relative difference value, relative ratio, etc.) on the basis of anangle of a slope of the equal quantization 1510. In this case, arelative value of the angle of the slope of the unequal quantization1520 and the slope of the equal quantization 1510 may be determinedusing a ratio (for example, see Table 6) of relative strength of thebeams a2 and a3 reported by the user equipment at the first step. Also,the slope of the unequal quantization 1520 may be determined on theassumption that the value of the beam angle θ₂ ¹ of a2 is a specificvalue 1530. In this case, the specific value 1530 may be determined as arelative value (for example, relative difference value, relative ratio,etc.) on the basis of the value of the beam angle θ₂ ¹ of a2, and therelative value of the specific value 1530 and the beam angle θ₂ ¹ of a2may be determined using a ratio (for example, see Table 6) of relativestrength of the beams a2 and a3 reported by the user equipment at thefirst step.

Also, in a non line of sight (Non-LOS) environment, the beam indexespreferred by the user equipment may not be continuous unlike theaforementioned example. In this case, the base station may configureeach beam pattern per beam index reported by the user equipment, whereinthe full beam patterns of the second step may be divided per beam indexreported by the user equipment. For example, if the beam indexesreported by the user equipment are θ₂ ¹, θ₅ ¹, the base station mayindividually configure the beam patterns such as

${\theta_{1}^{1} < \theta_{1,{{\ldots \mspace{14mu} \frac{m^{\prime}}{2}} - 1}}^{2} < \theta_{3}^{1}},{\theta_{4}^{1} < \theta_{\frac{m^{\prime}}{2},{\ldots \mspace{14mu} m^{\prime}}}^{2} < {\theta_{6}^{1}.}}$

In the same manner as the first step, the user equipment that hasreceived the beam patterns from the base station feeds index of itspreferred beam among the beam patterns transmitted from the base stationback to the base station. At this time, the user equipment may reportsignal strength of the reference signal corresponding to each index orchannel quality information (for example, CSI, CQI, PMI, etc.) to thebase station together with the index of the beam. Also, a type ofinformation on the beam reported by the user equipment at the secondstep may be different from a type of information on the beam reported tothe base station by the user equipment at the first step.

3) Jth Step

The operations of the base station and the user equipment at each of theaforementioned steps may be repeated in the same manner. In other words,at the Jth step, the base station configures beam patterns by usingfeedback information reported from the user equipment at the J−1th step.

In the meantime, the base station may indicate a feedback operation ofthe user equipment at each step by transmitting configurationinformation (hereinafter, referred to as ‘feedback configurationinformation’) on the feedback operation of the user equipment. Thisindication information may include at least one of the number of antennaports which are selected (or the number of beams), selection referenceof antenna ports (or beams), application or not of PMI(precoding/preferred matrix index), individual signal strength ofselected antenna port (or beam), individual signal quality (for example,CQI or CSI-RS based RSRP) of selected antenna port (or beam), signalquality when PMI is applied, and RI (Rank Indication/Index). At thistime, the base station may notify the user equipment of feedbackconfiguration information (feedback operations of the user equipment forthe respective steps may be the same as one another or different fromone another) on all the steps prior to the first step (for example,prior to the step S1101 in the example of FIG. 11) of the hierarchicalbeamforming scheme, or may notify the user equipment of feedbackconfiguration information per step prior to (for example, prior thesteps S1101, S1105 and S1109 in the example of FIG. 11) each step.Alternatively, the base station may notify the user equipment ofconfiguration information prior to the first step and prior to eachstep.

If the present invention is applied to the LTE-A system, an example ofallowing the base station to configure information to be fed back fromthe user equipment will be described. The base station may transmit atleast one type of CSI-RS configuration information to the user equipmentand indicate a type of feedback information to be fed back from the userequipment in accordance with each CSI-RS configuration. In other words,the base station may command the user equipment to feed backcorresponding antenna port index (or beam index) and correspondingsignal strength by selecting N₁ number of antenna ports (or beams) inaccordance with first CSI-RS configuration and to feed backcorresponding antenna port index (or beam index), PMI and CQI byselecting N₂ number of antenna ports (or beams) in accordance withsecond CSI-RS configuration. In this case, the user equipment may selectPMI from N2 Tx codebook.

Rank means the number of data streams (or layers) transmitted andreceived to and from the same resource at the same time. If a pluralityof beams or antenna ports are selected in the suggested method, at leastone stream to streams equivalent to the number of selected beams orantenna ports may be transmitted and received at the same time using thecorresponding beams or antenna ports. The base station may fix the ranksuch that the user equipment may configure feedback information byassuming the given rank. For example, if the rank is fixed to 1, eventhough the user equipment selects a plurality of beams or antenna ports,the user equipment may assume that the selected beams or antenna portsform one data stream. Alternatively, the base station may command theuser equipment to determine the rank and perform feedback. For example,in a state that the user equipment selects two beams (or antenna ports)and feeds index of the selected beams (or antenna ports) back andselects PMI from 2 Tx codebook and feeds the selected PMI back, althoughfeedback information corresponding to rank 1 and rank 2 includes indexesof the two beams (or antenna ports) and PMI, beam (or antenna port)index and the amount of PMI related feedback information may be reducedas follows considering features in the aforementioned LOS dominantenvironment. In other words, in case of rank 1, feedback informationsuch as (the highest index or the lowest index of the selected beams (orantenna ports)+PMI) may be configured. In case of rank 2, feedbackinformation such as (index of beam (or antenna port) selected at thefirst step+index of beam (or antenna port) selected at the second step)may be configured.

Alternatively, the number of selected antenna ports (or beams) which areindicated by the base station may be replaced with a maximum number (=M)of antenna ports (or beams) that may be selected by the user equipment.In this case, the codebook includes M×1 vectors, each of which may have1 (or 2) to M number of non-zero elements. For example, if the basestation commands the user equipment to select maximum three antennaports (or beams) and apply PMI, the codebook may be configured asexpressed by the following Equation 8 or 9, and PMI may be selected fromsuch a codebook.

$\begin{matrix}\left\{ {\begin{bmatrix}1 \\0 \\0\end{bmatrix},{\alpha_{1}\begin{bmatrix}1 \\1 \\0\end{bmatrix}},{\alpha_{2}\begin{bmatrix}1 \\{- 1} \\0\end{bmatrix}},{\alpha_{3}\begin{bmatrix}1 \\j \\0\end{bmatrix}},{\alpha_{4}\begin{bmatrix}1 \\{- j} \\0\end{bmatrix}},{\alpha_{5}\begin{bmatrix}1 \\1 \\1\end{bmatrix}},{\alpha_{6}\begin{bmatrix}1 \\{- 1} \\1\end{bmatrix}},{\alpha_{7}\begin{bmatrix}1 \\1 \\{- 1}\end{bmatrix}},{\alpha_{8}\begin{bmatrix}1 \\{- 1} \\{- 1}\end{bmatrix}}} \right\} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack \\\left\{ {{\alpha_{1}\begin{bmatrix}1 \\1 \\0\end{bmatrix}},{\alpha_{2}\begin{bmatrix}1 \\{- 1} \\0\end{bmatrix}},{\alpha_{3}\begin{bmatrix}1 \\j \\0\end{bmatrix}},{\alpha_{4}\begin{bmatrix}1 \\{- j} \\0\end{bmatrix}},{\alpha_{5}\begin{bmatrix}1 \\1 \\1\end{bmatrix}},{\alpha_{6}\begin{bmatrix}1 \\{- 1} \\1\end{bmatrix}},{\alpha_{7}\begin{bmatrix}1 \\1 \\{- 1}\end{bmatrix}},{\alpha_{8}\begin{bmatrix}1 \\{- 1} \\{- 1}\end{bmatrix}}} \right\} & \left\lbrack {{Equation}\mspace{14mu} 9} \right\rbrack\end{matrix}$

Also, if the present invention is applied to the LTE-A system, anexample of allowing the base station to configure a selection referenceof antenna ports (or beams) of the user equipment will be described. Thebase station may notify the user equipment of 8 Tx CSI-RS configurationand command the user equipment to select 1, 2 or 3 antenna ports (orbeams) of 8 CSI-RS antenna ports (or beams) and feed the selectedantenna ports (or beams) back. As a reference used when the userequipment selects antenna ports (or beams), antenna ports (or beams) ofwhich signal quality exceeds a first threshold value may be selected, orantenna ports (or beams) of which relative size as compared with maximumsignal quality exceeds a first threshold value may be selected. Whenfeeding the selected antenna ports (or beams) back, the user equipmentmay feed the result of the selected antenna ports (or beams), which aresequentially sorted in accordance with signal quality, back. At thistime, if the base station commands the user equipment to apply PMI, theuser equipment may select a precoding vector from the codebook, whichincludes precoding vectors of 2 Tx and 3 Tx, and may feed index of theselected precoding vector back. The codebook includes vectors ofdimension corresponding to the number of antenna ports (or beams) thatmay be selected by the user equipment. In other words, if the basestation commands the user equipment to select 1, 2, or 3 antenna ports(or beams) as described above, the user equipment selects one vectorfrom the codebook, which includes 2×1 vectors and 3×1 vectors, and feedsthe corresponding index back to the base station. If the number of 2×1vectors is N₂ and the number of 3×1 vectors is N₃, the index includesceil(log 2(N₂+N₃)) bits. In this case, Ceil(x) means the smallestnatural number of numbers greater than x. Alternatively, indexes ofceil(log 2(1+N₂+N₃)) bits may be fed back. In this case, one value (forexample, 0) in the indexes represents that one antenna port (or beam)has been selected. The codebook may be configured as expressed by thefollowing Equation 8 or 9. In the Equations 10 and 11, α_(k) is aconstant.

$\begin{matrix}\left\{ {1,{\alpha_{1}\begin{bmatrix}1 \\1\end{bmatrix}},{\alpha_{2}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\alpha_{3}\begin{bmatrix}1 \\j\end{bmatrix}},{\alpha_{4}\begin{bmatrix}1 \\{- j}\end{bmatrix}},{\alpha_{5}\begin{bmatrix}1 \\1 \\1\end{bmatrix}},{\alpha_{6}\begin{bmatrix}1 \\{- 1} \\1\end{bmatrix}},{\alpha_{7}\begin{bmatrix}1 \\1 \\{- 1}\end{bmatrix}},{\alpha_{8}\begin{bmatrix}1 \\{- 1} \\{- 1}\end{bmatrix}}} \right\} & \left\lbrack {{Equation}\mspace{14mu} 10} \right\rbrack \\\left\{ {{\alpha_{1}\begin{bmatrix}1 \\1\end{bmatrix}},{\alpha_{2}\begin{bmatrix}1 \\{- 1}\end{bmatrix}},{\alpha_{3}\begin{bmatrix}1 \\j\end{bmatrix}},{\alpha_{4}\begin{bmatrix}1 \\{- j}\end{bmatrix}},{\alpha_{5}\begin{bmatrix}1 \\1 \\1\end{bmatrix}},{\alpha_{6}\begin{bmatrix}1 \\{- 1} \\1\end{bmatrix}},{\alpha_{7}\begin{bmatrix}1 \\1 \\{- 1}\end{bmatrix}},{\alpha_{8}\begin{bmatrix}1 \\{- 1} \\{- 1}\end{bmatrix}}} \right\} & \left\lbrack {{Equation}\mspace{14mu} 11} \right\rbrack\end{matrix}$

Uplink

Considering a normal user equipment (user equipment having a smallnumber of antennas) in an uplink, the base station may perform maximumratio combining (MRC) by using the reference signal transmitted from theuser equipment. Accordingly, unlike the downlink, the uplink in the FDDsystem may be managed without considering reference signal overhead orfeedback overhead. However, in case of an environment, such as vehicle,train or building, that the user equipment may have a lot of antennas,complexity is increased due to overhead of the reference signaltransmitted from the user equipment and as the base station shouldperform channel estimation for each antenna of the user equipment.Accordingly, in this case, the suggested hierarchical beamforming schememay be applied to the uplink.

In order to perform hierarchical beamforming in the uplink, thehierarchical beamforming scheme should be managed UE-specifically fromthe first step in accordance with location of each user equipment,channel status or mobility. At this time, assuming Omni-antenna, it doesnot have a concept of a cell sector unlike the downlink, the userequipment may configure the beam of the first step cell-specifically tocover 360 degrees not 120 degrees.

Parameters which will be transferred from the base station to the userequipment to manage the uplink hierarchical beamforming scheme are asfollows.

1) Step size

2) The number of beam patterns

3) Value at the first step (value after the second step)

4) Preferred beam index and/or PMI

Referring to FIG. 11 again, in case of the uplink hierarchicalbeamforming, the transmitter may correspond to the user equipment, andthe receiver may correspond to the base station. The parameters for theuplink hierarchical beamforming scheme may respectively be notified fromthe base station (that is, receiver) to the user equipment (that is,transmitter) prior to the first step (for example, prior to the stepS1101 in the example of FIG. 11) and prior to each step (for example,prior to the steps S1101, S1105 and S1109 in the example of FIG. 11).

The base station may use a semi-static control channel (for example, RRCsignaling) or a dynamic control channel (for example, PDCCH) to indicatethe parameters to the user equipment. Alternatively, the base stationmay transfer these parameters to the user equipment together with RRCsignaling and PDCCH. In other words, since step size and the number ofbeam patterns are associated with hardware performance of the userequipment, the step size and the number of beam patterns may beindicated through RRC signaling, and an angle value θ that may be variedinstantaneously, beam index and PMI may be indicated through the PDCCH.At this time, a downlink control information (DCI) format of the PDCCHmay be configured as a new DCI format different from the conventionalformat and then transferred through a UE-search space (USS), or may betransmitted in such a manner that the parameters may be added to theconventional DCI format. Since the user equipment knows that indicationinformation is transmitted, if the indication information is transmittedthrough the USS, the user equipment may acquire the parameters withoutincreasing the number of blind decoding times for acquiring a DCIformat.

If the present invention is applied to the LTE-A system, an example ofallowing the user equipment to perform a hierarchical beamformingoperation by using a sounding reference signal (SRS) will be described.In order to perform the hierarchical beamforming operation in theuplink, the user equipment may first notify the base station that theuser equipment has a capability of managing hierarchical beamforming.Subsequently, the base station may designate two types of SRSconfigurations for the user equipment. The SRS transmitted from the userequipment on the basis of the first SRS configuration and the second SRSconfiguration is generated by applying different weight values to Mnumber of antennas. For example, antenna weight values which are usedwhen 4 SRS antenna ports (or beams) are transmitted in accordance withthe first SRS configuration and when 4 SRS antenna ports (or beams) aretransmitted in accordance with the second SRS configuration mayrespectively be Ω={w₁, w₂, w₃, w₄} and Θ={v₁, v₂, v₃, v₄}.

In this case, n≠Θ, and w_(k) and v_(k) respectively mean weight vectorsof M×1 size used for the kth SRS antenna port (or beam) transmitted inaccordance with the first and second SRS configurations. In other words,the base station that has received the SRS transmitted from the userequipment in accordance with the first SRS configuration transfers themeasurement result of SRS to the user equipment, and the user equipmenttransmits the SRS by determining the weight value of the SRS antennaport (or beam) based on the second SRS configuration on the basis of themeasurement result of SRS. Preferably, the user equipment may know a setΩ of weight vectors which will be used for SRS for transmission inaccordance with the first SRS configuration and a set Θ₁, . . . , Θ_(L)of weight vectors which will be used for SRS for transmission inaccordance with the second SRS configuration. If the base stationmeasures the SRS transmitted from the user equipment in accordance withthe first SRS configuration and notifies the user equipment that SRSantenna port (or beam) having the best signal quality is #p (antennaport (or beam) which is absolute or relative), the user equipment usesweight vectors of Θ_(#p) for SRS which will be transmitted in accordancewith the second SRS configuration. It is preferable that the basestation dynamically notifies the user equipment of the measured resultof the SRS based on the first SRS configuration, through DCI. Thecorresponding DCI may include one or more SRS antenna port (or beam)indexes. Alternatively, the corresponding DCI may include a bitmap of asize equivalent to the number of antenna ports (or beams) of the firstSRS configuration. The user equipment determines a weight vector of theSRS which will be transmitted by the second SRS configuration on thebasis of the SRS antenna port (or beam) expressed as ‘1’ in the SRSantenna port (or beam) indexes.

The base station may notify the user equipment of an antenna port (orbeam) which will be used for the uplink and/or a precoding vector whichwill be applied between the antenna ports (or beams) by measuring theSRS transmitted in accordance with the second SRS configuration. Forexample, after measuring four SRS antenna ports (or beams) transmittedfrom the user equipment in accordance with the second SRS configuration,the base station may include any one precoding vector index belonging toa codebook expressed by the following Equation 12 in the DCI format (forexample, DCI format 0 or 4) used for PUSCH scheduling.

                                     [Equation  12] $\begin{Bmatrix}{{{{{{{{{{{{\begin{bmatrix}1 \\0 \\0 \\0\end{bmatrix}\begin{bmatrix}0 \\1 \\0 \\0\end{bmatrix}}\begin{bmatrix}0 \\0 \\1 \\0\end{bmatrix}}\begin{bmatrix}0 \\0 \\0 \\1\end{bmatrix}}\begin{bmatrix}1 \\1 \\0 \\0\end{bmatrix}}\begin{bmatrix}1 \\{- 1} \\0 \\0\end{bmatrix}}\begin{bmatrix}1 \\j \\0 \\0\end{bmatrix}}\begin{bmatrix}1 \\{- j} \\0 \\0\end{bmatrix}}\begin{bmatrix}1 \\0 \\1 \\0\end{bmatrix}}\begin{bmatrix}1 \\0 \\{- 1} \\0\end{bmatrix}}\begin{bmatrix}1 \\0 \\j \\0\end{bmatrix}}\begin{bmatrix}1 \\0 \\{- j} \\0\end{bmatrix}}\begin{bmatrix}1 \\0 \\0 \\1\end{bmatrix}} \\{{{{{{{{{{{{{\begin{bmatrix}1 \\0 \\0 \\{- 1}\end{bmatrix}\begin{bmatrix}1 \\0 \\0 \\j\end{bmatrix}}\begin{bmatrix}1 \\0 \\0 \\{- j}\end{bmatrix}}\begin{bmatrix}0 \\1 \\1 \\0\end{bmatrix}}\begin{bmatrix}0 \\1 \\{- 1} \\0\end{bmatrix}}\begin{bmatrix}0 \\1 \\j \\0\end{bmatrix}}\begin{bmatrix}0 \\1 \\{- j} \\0\end{bmatrix}}\begin{bmatrix}0 \\1 \\0 \\1\end{bmatrix}}\begin{bmatrix}0 \\1 \\0 \\{- 1}\end{bmatrix}}\begin{bmatrix}1 \\1 \\0 \\j\end{bmatrix}}\begin{bmatrix}1 \\1 \\0 \\{- j}\end{bmatrix}}\begin{bmatrix}0 \\0 \\1 \\1\end{bmatrix}}\begin{bmatrix}0 \\0 \\1 \\{- 1}\end{bmatrix}}\begin{bmatrix}0 \\0 \\1 \\j\end{bmatrix}} \\{{{{\begin{bmatrix}0 \\0 \\1 \\{- j}\end{bmatrix}\begin{bmatrix}1 \\1 \\1 \\1\end{bmatrix}}\begin{bmatrix}1 \\{- 1} \\1 \\1\end{bmatrix}}\begin{bmatrix}1 \\1 \\{- 1} \\1\end{bmatrix}}\begin{bmatrix}1 \\1 \\1 \\{- 1}\end{bmatrix}}\end{Bmatrix}$

This codebook should be known by the base station and the userequipment. If any one precoding vector index belonging to the codebookis included in the DCI format, additional information on the antennaport (or beam) index may not be included in the DCI format. The exampleof the above codebook includes precoding vectors of which the number ofnon-zero elements is 1, 2 and 4. Each of the vectors belonging to thecodebook may be multiplied by a specific constant value.

Hereinafter, the simulation result obtained by comparison betweenconventional beamforming (CBF) and hierarchical beamforming (HBF) willbe described. Hereinafter, switched beamforming in the simulation resultmeans the conventional beamforming. In other words, as described above,the step for beamforming includes one step only, and if the base stationgenerates a total of M number of beam patterns, the user equipmentreports only one beam pattern to the base station, and the base stationgenerates a beam pattern having an angle corresponding to a beam patternindex reported by the user equipment. In case of HBF, it is assumed thattwo steps for beamforming are configured, and 16 beam patterns aregenerated using 64 antennas. Hereinafter, in the simulation result, avertical axis represents capacity, and means bits that may betransmitted per time/Hz. A horizontal axis represents a signal-to-noiseratio (SNR).

FIG. 16 is a graph illustrating a simulation result based on ahierarchical beamforming scheme according to the present invention.

Referring to FIG. 16, the HBF provides throughput gain of about 2 dB to8 dB on the assumption of the same expected capacity as compared withthe CBF in accordance with the simulation result in a LOS environment.Alternatively, the HBF provides gain of about 0.5˜2 bit/s/Hz in the sameSNR environment (−5˜20 dB).

FIG. 17 is a graph illustrating another simulation result based on ahierarchical beamforming scheme according to the present invention.

Referring to FIG. 17, a simulation result is obtained on the assumptionof high antenna correlation (it is assumed that correlation betweenantennas is 0.9) in a non-LOS environment. If rank is 2, the HBF assuresthroughput more excellent than that of the CBF. However, if rank is 1,the HBF obtains the same throughput as that of the CBF. Since beamadaptation for each rank is hierarchically managed in spatialmultiplexing and beam for each rank is precoded using the codebook, theHBF may obtain such throughput gain if rank is 2.

FIG. 18 is a graph illustrating other simulation result based on ahierarchical beamforming scheme according to the present invention.

Referring to FIG. 18, a simulation result is obtained on the assumptionof low antenna correlation (it is assumed that correlation betweenantennas is 0) in a non-LOS environment. Since independence of eachchannel for performing spatial multiplexing is assured if antennacorrelation is low, throughput corresponding to rank 2 is more excellentthan that corresponding to rank is 1. However, since antenna correlationis low, a pattern of a beam is not generated normally, and correlationbetween the beam generated at the second step and the beam generated atthe first step or subset concept is reduced. Accordingly, throughput ofthe HBF is more degraded than that CBF as shown in FIG. 18.

System to which the Present Invention May be Applied

FIG. 19 is a block diagram illustrating a wireless communication systemaccording to one embodiment of the present invention.

Referring to FIG. 19, the wireless communication system includes a basestation 190 and a plurality of user equipments 200 located within thebase station 190.

The base station 190 includes a processor 191, a memory 192, and a radiofrequency (RF) unit 193. The processor 191 may be configured toimplement functions, procedures and/or methods suggested in the presentinvention. Layers of a radio interface protocol may be implemented bythe processor 191. The memory 192 is connected with the processor 191and stores various kinds of information related to the operation of theprocessor 191. The RF unit 193 is connected with the processor 191 andtransmits and/or receives a radio signal.

The user equipment 200 includes a processor 201, a memory 202, and aradio frequency (RF) unit 203. The processor 201 may be configured toimplement functions, procedures and/or methods suggested in the presentinvention. Layers of a radio interface protocol may be implemented bythe processor 201. The memory 202 is connected with the processor 201and stores various kinds of information related to the operation of theprocessor 201. The RF unit 203 is connected with the processor 201 andtransmits and/or receives a radio signal.

The memory 192, 202 may be located inside or outside the processor 191,201, and may be connected with the processor 191, 201 by various meanswhich are well known. Also, the base station 190 and/or the userequipment 200 may have a single antenna or multiple antennas.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with another claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

The embodiments according to the present invention may be implemented byvarious means, for example, hardware, firmware, software, or theircombination. If the embodiment according to the present invention isimplemented by hardware, the embodiment of the present invention may beimplemented by one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, etc.

If the embodiment according to the present invention is implemented byfirmware or software, the embodiment of the present invention may beimplemented by a type of a module, a procedure, or a function, whichperforms functions or operations described as above. A software code maybe stored in a memory unit and then may be driven by a processor. Thememory unit may be located inside or outside the processor to transmitand receive data to and from the processor through various means whichare well known.

It will be apparent to those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

INDUSTRIAL APPLICABILITY

Although various methods according to the present invention have beendescribed based on the 3GPP LTE system, they may equally be applied tovarious wireless access systems in addition to the 3GPP LTE system.

1. A method for performing uplink hierarchical beamforming at a userequipment (UE) in a wireless access system, the method comprising:receiving, by the UE from eNode B, first reference signal configurationinformation and second reference signal configuration information at auser equipment; transmitting, by the UE to the eNodeB, first referencesignal according to the first reference signal configuration through afirst plurality of antenna port using first weight vectors;transmitting, by the UE to the eNodeB, a second reference signalaccording to the second reference signal configuration through a secondplurality of antenna port using second weight vectors, wherein thesecond weight vectors are determined based on feedback information thatincludes at least one of index of antenna port or index of precodingvector.
 2. The method according to claim 1, wherein the first weightvectors include one or more weight vectors, each of which ispredetermined for each of the first plurality of antenna port.
 3. Themethod according to claim 2, wherein the second weight vectors aredetermined among a candidate weight vectors.
 4. The method according toclaim 3, wherein the second weight vectors include one or more weightvectors predetermined for the at least one of index of antenna port. 5.The method according to claim 4, wherein the candidate weight vectorsinclude a plurality of weight vectors, each of which is predeterminedfor each of the second plurality of antenna port.
 6. The methodaccording to claim 1, the first weight vectors and the second weightvectors are different each other.
 7. The method according to claim 1,the first reference signal and the second reference signal correspondsto sounding reference signal (SRS).
 8. A user equipment (UE) performinguplink hierarchical beamforming in a wireless access system, the UEcomprising: a receiver configured to receive a signal from eNode B; atransmitter configured to transmit a signal to the eNodeB; and aprocessor configured to control a receiver for receiving first referencesignal configuration information and second reference signalconfiguration information at a user equipment, a transmitter fortransmitting first reference signal according to the first referencesignal configuration through a first plurality of antenna port usingfirst weight vectors and transmitting a second reference signalaccording to the second reference signal configuration through a secondplurality of antenna port using second weight vectors, wherein thesecond weight vectors are determined based on feedback information thatincludes at least one of index of antenna port or index of precodingvector.
 9. The UE according to claim 8, wherein the first weight vectorsinclude one or more weight vectors, each of which is predetermined foreach of the first plurality of antenna port.
 10. The UE according toclaim 9, wherein the second weight vectors are determined among acandidate weight vectors.
 11. The UE according to claim 10, wherein thesecond weight vectors include one or more weight vectors predeterminedfor the at least one of index of antenna port.
 12. The UE according toclaim 11, wherein the candidate weight vectors include a plurality ofweight vectors, each of which is predetermined for each of the secondplurality of antenna port.
 13. The UE according to claim 8, the firstweight vectors and the second weight vectors are different each other.14. The UE according to claim 8, the first reference signal and thesecond reference signal corresponds to sounding reference signal (SRS).